This invention relates to an improved process for the removal of residual monomer from aqueous dispersions of homo- and co-polymers of vinyl chloride.
The manufacture of homo- and copolymers of vinyl chloride by aqueous emulsion polymerization is well known. See, for example, monograph by Kainer, "Polyvinylchlorid and Vinylchlorid-Mischpolymerisate", Springer publishers, Berlin/Heidelberg/New York, 1965, pp. 34 et seq.
In the emulsion homo- and copolymerization of vinyl chloride, the polymerization generally is not continued to a quantitative conversion of the monomers. In fact, in many cases, the polymerization is terminated intentionally at a low degree of conversion. In all instances, considerable amounts of residual monomers remain in the thus-produced aqueous polymer dispersions.
The removal of a large portion of the residual monomers occurs upon expansion and/or evacuation, usually in the polymerization reactor. Vinyl chloride removed under vacuum is reintroduced into the gasometer. In spite of these vinyl recovery steps, the aqueous dispersion still contains about 1-2% by weight of monomer, based on the polymer. During the spray drying of these dispersions obtained from the emulsion polymerization more than 95% by weight of this residual monomer escapes into the atmosphere mixed with the dryer exhaust air. Spray-dried polyvinyl chlorides thus have residual monomer contents in the order of only 10-300 p.p.m.
However, as a result of ever-increasing efforts to reduce environmental pollution, the problem has arisen of decreasing the heretofore considerable vinyl chloride contents of the dryer exhaust air. To achieve this, it is necessary to substantially free the polyvinyl chloride dispersions of monomers prior to the drying procedure.
It is known that readily volatile monomers can be removed from synthetic resin dispersions by storing the dispersion in large expansion tanks. See German Pat. No. 1,248,943, column 2, lines 4-6. However, to employ this mode of operation requires an enormous capital investment in apparatus.
It is also known to conduct dispersions of polymers, including polyvinyl chloride, countercurrently to rising inert gases in spray adsorbers, thereby freeing the dispersions from readily volatile compounds. See German Unexamined Laid-Open application DOS 2,162,860. This mode of operation also requires a huge capital investment in apparatus.
The steam distillation of polymer dispersions for the purpose of removing readily volatile components is also known. See German Pat. No. 1,248,943, column 2, lines 24-28. However, this mode of operation cannot be readily employed with polymer dispersions obtained by emulsion polymerization because such dispersions, due to their emulsifier content, have such a low surface tension that excessive foaming occurs during a steam distillation.
In a known (German Pat. No. 1,248,943) batch process, the foam produced during the steam distillation of dispersions having a surface tension of below 40 dynes per centimeter is broken up by subjecting the thus-formed steam-dispersion foam mixture to a rapid pressure drop at flow velocities of more than 100 m./sec. The thus-defoamed dispersion is then reintroduced into the steam distillation from the separator, i.e., it is recycled. As disclosed in German Pat. No. 1,248,943, column 5, lines 19-22, such a process can be conducted continuously only employing several units connected in series in a cascade arrangement. Such an arrangement obviously requires a huge investment in capital equipment. Also, such a series arrangement of several units represents a large mechanical load on the dispersions, which have a high solids content and are low in emulsifier, as they are exclusively manufactured in modern large-scale technical processes. As a result, in a most undesirable way, depositions of sediment and coagulate are formed from the poorly stable dispersions in the pipelines between the degasification vessels.
As disclosed in Fed. Rep. of Germany Patent application P 24 40 957.2, corresponding to U.S. Pat. No. 4,007,022 such difficulties are overcome in a continuous process for the removal of monomeric impurities from aqueous dispersions of homo- and copolymers of vinyl chloride having a surface tension of below 60 dyn/cm. by treatment of the dispersions with steam and avoiding foam formation by rapidly lowering the pressure at high flow velocities. In that process, a stream of the dispersion is mixed continuously with steam in an elongate mixing zone, via., a flow pipe, at a rate such that the flow velocity of the mixture therein is 1-20 m./sec., and at a pressure in the flow pipe of 150-600 torr. A stream of the mixture of steam and dispersion is withdrawn from the flow pipe at a speed of 30-100 m./sec., and fed to an evacuated degasification zone, i.e., a degasification vessel, maintained at a pressure which is 30-200 torr lower than the pressure in the flow pipe. A stream of the degasified mixture is withdrawn continuously from the degasification vessel.
It has now been found that surprisingly very much lower residual monomer concentrations can be attained by the process of this invention than are attainable by the continuous steam treatment described in U.S. Pat. No. 4,020,032.